An electronic semiconductor device of the kind outlined above is a composite structure typically adapted to accommodate an integrated circuit thereon. As is known, an integrated circuit is implemented on a die (chip) of a semiconductor material having a surface area of a few square millimeters, and requires, for its connection to an external circuit such as a printed circuit on a board made of an insulating material, a special holding structure which also enables the circuit to be interconnected electrically. A known structure that fills this demand is basically comprised of a plastics body enclosing the chip, the chip being connected, as by thin wire leads soldered to purposely provided metallized areas of its surface, to corresponding terminal conductors led out of the plastics body. In the instance of power integrated circuits, that is, devices designed to operate on comparatively large currents and accordingly liable to become heated to a considerable extent, such structures further include a metal plate whereby the chip secured thereon can transfer the heat developed during its operation to the outside.
The manufacture of the last-named structures provides for the plate to be formed from sheet metal, e.g., a copper sheet, by a blanking process. The chip is then attached to the metal plate either by a soldering process using a low melting point solder, such as a lead-tin solder, or by gluing using a suitable adhesive, such as an epoxy-based adhesive. Thereafter, a set of metal strips intended to form the device pins or terminal conductors, and which have been blanked off sheet metal but are still held together by interconnecting bridge portions, are mounted on the plate in an electrically isolated fashion therefrom. Thin wires, usually golden wires, are then soldered with one end to the metallized areas of the chip using a low melting point solder, and with the other end, to the ends of the metal strips by a process wherein heat and ultrasound are applied simultaneously. Subsequently, the whole is placed in a suitable mold, into which a plastics material, such as a thermosetting epoxy resin, in a liquefied state is poured. A resin curing step yields then a solid plastics body encapsulating the above-described elements but for one face of the metal plate and part of the metal strips, i.e., the device pins, and the interconnecting bridges therebetween. The latter are then removed as by blanking to yield the finished electronic device.
In the instance of devices intended for surface mounting, that is, for attachment to a printed circuit board by soldering their pin ends to specially arranged metallic areas on the board surface, the pins are bent such that their end portions will all lie in the same plane as one face of the device. This face may alternatively be the exposed surface of the metal plate or the surface of the plastics body opposite to it.
The bend direction of the pins, and hence the face of the device which is to contact the printed circuit board, is selected to suit the device's own heat dissipation requirements in a specific circuit application.
Where the amount of heat to be so dissipated is relatively small, or where a broad metal surface is available on the board for heat dissipation purposes, the pins are bent to cause the exposed surface of the plate to contact the board, specifically a metallic area to which the plate is soldered simultaneously with the pin soldering operation.
With a larger amount of heat to be dissipated, or where it is undesirable or impossible to use a broad surface of the board for heat dissipation, the pins are bent in the opposite direction and a large-size heat sink is attached to the exposed surface of the plate, which is positioned in a plane remote of the plane at which the electronic device contacts the board.
The last-described situation is illustrated by FIG. 1 of the drawings that accompany this specification. For securing the heat sink, indicated there at 10, it is common practice to use a clip 11 affixed to the board 12 and effective to push, with a middle portion 11b thereof, the heat sink 10 centrally against the board, and therefore, against the metal plate 13a of the intervening electronic device, designated 13. In the example shown, the clip 11 is a shaped strip of spring steel having enlarged ends 11a arranged to fit in respective openings 12a of appropriate shape formed in the board 12.
This mounting arrangement has, however, some drawbacks. In particular, it requires that two areas of the board be made available for applying the clip, which detracts from the compactness of the printed circuit as a whole and possibly from the quality of the electric contact between the device pins, designated 13b, and the printed circuit, shown at 12b, because the pressure exerted on the heat sink 10 by the clip 11 will be transferred to the solder spots, not shown, of the pins 13b in the purposely provided areas 12c of the printed circuit.